1. Field of the Invention
The present invention relates to an inertial measurement unit (IMU) and, more particularly, to a single IMU which measures angular rate and linear acceleration for use in a six degrees of freedom, three axis orthogonal system through the use of a single counterbalanced low inertia mechanical oscillator.
2. Description of the Prior Art
Existing strap down inertial measurement units typically consist of a cluster of separate gyros and accelerometers of complex electro-mechanical and/or electro-optical design. These IMUs were originally designed to meet the high performance requirements of an aircraft navigation system with operation times measured in hours. Application of these gyros and accelerometers to flight control IMU's for short duration flight, such as required for expendable weapons which operate in times measured in minutes, has resulted in systems with excessive size, weight, power, and cost, and with inadequate environmental capabilities. Remember that the existing strap down IMUs typically require a separate gyro and accelerometer for each axis within a three axis orthogonal system.
Some systems have been proposed which reduce the number of gyros and accelerometers by using one specialized sensor for sensing two axes. Typically, the specialized sensor is placed upon a spinning shaft. As the multiple sensor is capable of sensing two of the three axes within an orthogonal system, there still remains the requirement for two sets of the sensors as well as the need for motors and bearings to accommodate the spinning axes. This also results in a redundant axis which adds size, weight, and cost.
Another approach to an inertial measurement unit for measuring the specific force and angular velocity of a moving body utilizes an orthogonal triad of rotating accelerometers. This system reduces the number of components required to provide an IMU, but there still remains the requirement for spinning three accelerometers around the three orthogonal axes with the accompanying requirement for bearings and spin motors and resolvers. It has also been suggested that the spinning accelerometers may be vibrated in an oscillating manner instead of spinning around the orthogonal axes.
A breakthrough improvement over the arrangements just described may be found in a copending patent application, Ser. No. 045,045, filed May 1, 1987, by Robert E. Stewart, entitled "A Miniature Inertial Measurement Unit" now U.S. Pat. No. 4,841,773 which is assigned to the same assignee as the present invention. This inertial measurement unit utilizes a single rotor design with six accelerometers mounted at 60.degree. apart on the rim of a paddle-wheel like body. An AC voltage is used to drive piezoelectric strips mounted on the webs of the paddle-wheel like body resulting in a simple harmonic oscillation of the body. The design produces a three axis orthogonal measurement system from but one rotating body. This design works well for some applications but has some difficulty with angular rate performance in the presence of random vibration.